Electric circuit elements



OXYGEN- IMPERVIOUS l ELECTRODE \I| PERVASIVE ELECTRODE will!" mulii May 23, 1967 IO `l 2 INVENTOR.

BY. v

United States Patent O 3,321,683 ELECTRIC CIRCUIT ELEMENTS William A. Tatem, Avon, N.Y., assignor to Sprague Electric Company, North Adams, Mass., a corporation of Massachusetts Filed June 1, 1965, Ser. No. 465,823 12 Claims. (Cl. 317-258) This application is a continuation-in-part of copending application S.N. 248,495 filed Dec. 31, 1962, now abandoned. This invention is concerned with electric circuit elements and more particularly with ceramic capacitors.

It is well-known in the art to form a capacitor by (l) firing a ceramic body to maturity in air, (2) reducing this body by firing the same in a reducing atmosphere, and (3) reoxidizing the surface of the body while firing-on the silver capacitor electrodes. By the first step, the ceramic acquires the characteristics of an insulator or a dielectric; the second step results in the body exhibiting semi-conducting properties and by the third step, the surface of the semi-conducting body, including that beneath the silver electrodes, is converted to a thin insulating lm.

The resulting units, therefore, consist of a semi-conducting body separating two extremely thin dielectric layers which are in intimate contact with two silver electrodes. This arrangement defines two capacitors connected in series by the semiconductor. The capacitance is about half what it would be if there were only one dielectric layer.

Several methods have been suggested for eliminating one of the dielectric layers, thereby effectively doubling the capacitance. One suggestion was to physically abrade one of the dielectric layers, and make Contact through the semiconductor to the other dielectric. It was also suggested that a layer of low melting point solder be applied to one of the silver electrodes. The solder was believed to destroy the juction between the electrode and the semiconductor to the extent that the dielectric layer beneath the electrode was shorted out.

It is suggested that these methods would be time-consuming and unreliable. They, therefore, do not oder a satisfactory solution to the problem.

It is an object of the present invention to overcome the foregoing and related disadvantages.

It is another object to present a ceramic capacitor having an extremely thin dielectric and two dissimilar electrodes.

It is still another object to control the formation of the dielectric in a ceramic capacitor.

These and other objects will best be understood from the following description of the specific embodiments when read in conjunction with the accompanying drawing, in which:

FIGURE 1 is a view, in side elevation, of a ceramic capacitor according to the present invention;

FIGURE 2 is a sectional view taken on line 2-2 of- FIGURE l;

FIGURE 3 is a sectional view of two capacitors connected in parallel.

In accordance with the present invention the above objects are achieved by forming a ceramic capacitor comprising a ceramic body separating an oxygen-impervious electrode and an oxygen-pervasive counterelectrode. The surface of the ceramic body underlying or adjacent the counterelectrode is in an oxidized state and the ceramic body underyling or adjacent to the oxygen impervious electrode is in a reduced state.

In general the process of the present invention isv as follows:

A single oxygen impervious electrode is applied to a ceramic body. The body may be either in the green state or already fired to maturity. This unit is then red lCe in a reducing atmosphere to convert the ceramic to a semiconducting body throughout. Finally, after applying an oxygen-pervasive counter/electrode kthe unit is heat- Ved in an oxidizing atmosphere. The ring temperature during the nal heating step in an oxidizing atmosphere must be high enough for oxygen to pervade one electrode but not high enough for it to pervade the other electrode. Moreover, the temperature must be high enough to effect oxidation of the ceramic beneath the oxygen pervasive electrode. The resulting unit is a pair of electrodes separated by a relatively thick semiconducting ceramic having a relatively thin, highly insulating layer underlying the electrode which permitted transmission of oxygen during the final heating step.

Applicant does not desire to be bound in any way by theory, however, it is believed that the principle of the present invention can be explained in the following manner. It has been determined that the noble metals and certain of their alloys are impervious to the transmission of oxygen up to some particular temperature, which is different for each metal and alloy. In the case of palladium no gas other than hydrogen can pass through the heated metal lattice until a particular threshold temperature is reached. Above this temperature, which is comparatively high, it is known that at least oxygen is also transmitted. Silver, by way of comparison, has an extremely low threshold temperature. In addition, the solubility of oxygen in silver is very high, especially at temperatures near the melting point of the metal. The solubility of oxygen under 1 atmosphere pressure at the melting point of silver is 20 volumes per volume of metal.

The other nobel metals, gold, platinum, osmium, iridium, rhodium and ruthenium have so called oxygen threshold temperatures different from those of palladium and silver.

The threshold temperature of silver is so low and its ability to dissolve oxygen is so great that it stands apart from the other noble metals as far as the purposes of the present invention are concerned. Thus, silver will always be an oxygen-pervasive electrode.

As used herein the term oxygen-pervasive electrode means an electrode having the property of permitting oxygen to pervade it and be transmitted thereby at a temperature sufficiently high to effect oxidation of a reduced ceramic within a reasonable time.

The term oxygen-impervious electrode means an electrode having the characteristic of being impervious to oxygen at temperatures at which its counterelectrode would be oxygen-pervasive.

The threshold temperatures for the electrodes of the present invention cannot be stated with exact precision because certain variables influence the rate of oxygen transmission. For example, the electrode thickness, if anywhere in the region of 5 mils, will not transmit any sensible quantity of oxygen within a reasonable length of time. Thus, the present invention will only be concerned with electrode thicknesses of up to about 1 mil. Moreover, the oxygen pressure must be high enough to oxidize a reduced ceramic within a reasonable length of time for commerical units.

Keeping such limitations in mind, the following are the respective oxygen threshold temperatures for the platinum group metals and gold. Clearly above these temperatures the electrodes would transmit oxygen.

Referring to lthe drawing, FIGURE l shows a ceramic disc capacitor Iof the present invention and FIGURE 2 shows a sectional View taken on the line 2-2 of FIGURE 1. In the drawing, represents the reduced, semiconducting region of the ceramic, 11 the oxygen impervious electrode, 12 the oxidized dielectric layer and 13 the oxygen-pervasive counterelectrode.

Capacitors made according to this invention are described in the following examples.

Example I (Oxygen-impervious electrode is palladium) Nine green barium titanate discs of about 1A diameter and about 30 mils thick were prepared. A thin lilm of a palladium paint was applied to one side of discs 1, 2 and 3 and to both sides of discs 4, 5 and 6. No electrodes were applied to discs 7, 8y and 9 until later in the process. The nine discs were then red to maturity in air at a temperature of about 2410 F. for a period of about 1% hours. At this stage the ceramic discs had the properties of a dielectric. The discs were then reduced in hydrogen at a temperature of about 2240 F. for a period of about l hour. Extreme care was exercised to avoid exposing the palladium to hydrogen below about 950 F. It has been determined that if there is exposure at certain temperatures below this, the palladium electrode will not adhere to the ceramic surface. At this stage in the process, the reduced discs exhibited semiconducting characteristics throughout. After cooling, a silver electrode paint was applied to all the barren faces of the discs. The nine units were then red in air at a temperature of about 1450 F. for a period of about 18 minutes.

Thereafter the units were aged for about 24 hours and then checked for capacitance and dissipation factor at 1 kc., 25 C. and 0.5 VRMS.

The following table is a summary of the pertinent data obtained.

ceramic was past. One representative disc of each group was selected. A thin lm of silver paint was applied to the bar-ren face of the disc having one platinum electrode red thereon. A thin lm of silver paint was applied to, each of the barren faces of the nonelectroded disc. The three representative discs were then red in air at a temperature of about l000 F. for about 18 minutes.

Thereafter the units were aged for about 24 hours `and then checked for lcapacitance land dissipation factor at 1 kc., 25 C. `and 0.5 VRMS. The electrode areas were approximately equal.

Electrodes Capacitance (upf.) D .1?. (Percent) Short;

Pt/Ag Capacitance ratio Ag [Ag-1.91

The approximate resistance of the Pt/Pt unit was less than 10 ohms.

It will be noted that the platinum electrode would not permit oxygen transmission at the nal oxidation ternperature of 1000 F. since this is below its threshold temperature of about 1100 F.

FIGURE 3 of this drawing illustrates another embodiment of the present invention. Here, the palladium electrode 11 is sandwiched between two titanate layers and two silver counterelectrodes 13 which are connected through a common lead-wire 1S. The titanate layers 10, including the surface thereof adjacent to the palladium electrode 11 is in a reduced state and has the properties of a semiconductor. The surfaces 12 underlying or adjacent to the silver electrodes 13, are in an oxidized state and have the properties of an insulator. Lead-wire 14 is attached to the palladium electrode. The counterelectrodes 13 can be placed in electrical communication by *The approximate resistance of units 4, 5 and 6 is less than 10 ohms.

Average capacitance/Area (a/in?) of Ag/Pd:0.506. Average caApaiItance/Area (lli/in?) of Ag/Ag:0.252. Ratio of -ATzZ It will be noted in the preceding example that the palladium electrode would not permit oxygen transmission at the reoxidation temperature of 1450 F. since this is considerably below its threshold temperature of about 1900 F.

Example Il (Oxygen-impervious electrode is platinum) A plurality of green barium titanate discs of -about 1/2 diameter and 20-30 mils thick were prepared. A thin lnl less than l mil thick, of platinum paint was applied to one side of 7 discs. A thin lm of the same thickness of platinum was applied to both sides of 8 discs. About 12 discs had no electrodes applied thereto. These discs were fired to maturity in air at a temperature of about 2400 F. for a period of about 1 hour. At this stage the ceramic discs had the properties of .a dielectric. Thereafter, the discs were reduced in hydrogen at a temperature of about 23 00 F. for l hour and -cooled in the absence of oxygen until danger of oxidation of the electrodes and any convenient means. For example, instead of using a common lead-wire, a continuous `counterelectrode bridging the unit can be employed. This can be applied by spraying, dipping, etc.

Units such as those illustrated by FIGURE 3 can be formed by combining tWo of the units of FIGURE 2 so that the palladium electrodes are placed in electrical communication with one another. This can be accomplished by fusing the two together, bonding them by use of a conductive paste, etc.

The following process may also beemployed:V

Example III A thin lrn of palladium electrode paint is sandwiched between two green barium titanate layers and this body is fired to maturity in air at a temperature of about 2410 F. for a period of about 11A hours. Taking care not to expose the palladium to hydrogen at a temperature below about 950 F., this unit is then reduced in hydrogen at a temperature of about 2240 F. for a period of about 1 hour. After cooling, a silver electrode paint is applied to opposite sides of the unit. This is then red in air at a temperature of about 1450 F. for about 15 minutes.

An electrode is affixed tothe pall-adium electrode and a common lead-wire is Iconnected to the two silver electrodes. Units such as these will have about four times the capacitance of a silver-silver electroded ceramic capacitor of the same electrode area.

It will be understood that the other noble metals can be employed as were platinum and palladium in the preceding examples, taking into consideration the particular threshold `temperature of the electrode material selected. It will also be appreciated that silver can be replaced as the oxygen-pervasive electrode by using, for example, platinum with ruthenium or with any other of the metals having a higher threshold temperature than platinum. Likewise, gold or osmium may be the oxygen-pervasive electrode with any of the other metals having a higher threshold temperature being the oxygen-impervious electrode.

The noble metal selected to be the oxygen-impervious electrode must be free of any material which would give up oxygen or which would permit the diffusion of oxygen through this electrode during the reoxidation or final oxidation step. It is to be understood, however, that t-he electrode need not be 100% pure vnoble metal. Certain alloys impervious to oxygen can be employed with advantage. For example, palladium alloyed with up to about 30% by weight of silver yields an extremely durable electrode which is still impervious to oxygen below about the threshold temperature of pure palladium. Thus, the noble metals may be alloyed with other metals so long as the resulting alloy will still be effective as an oxygen-impervious electrode at temperatures at which the counterelectrode is oxygen-pervasive.

The counterelectrode, on the other hand, can be any electrode material which has the essential properties of good conductivity and which also will permit oxygen to pervade it. In addition to silver, and t-he various noble metal combinations disclosed, certain materials can be -added to the noble metals which will permit the transmission of oxygen at a temperature lower than the threshold temperature of the pure metal. For instance, one commercial palladium paint contains a glass frit. These materials, e.g. glass frit, permit a reduced ceramic surface adjacent such an electrode to be oxidized either by allowing oxygen to pass through or by giving up oxygen. Thus, materials such as glass frit may be added to any electrode which tends to inhibit the passage of oxygen in order to convert it to a more effective counterelectrode.

The electrode materials may originate from any convenient precursor, for example, a paste, paint, organometallic resinates, etc. It may be applied in any suitable manner, e.g. painting, evaporative deposition, screening, etc.

The ceramic compositions contemplated for use herein include broadly the titanates including, the alkaline earth metal titanates, particularly barium titanates and modified barium titanates. The compositions may contain any of the prior art additives designed to improve the electrical or physical characteristics of the ceramic. Examples of such additives are excess titania, niohium oxide, niobates, zirconates, stannates, etc.

The oxidizing atmosphere can be lair, oxygen or any other gas mixture which will result in oxidation. The reducing atmosphere can be hydrogen, carbon monoxide, forming gas (80% nitrogen 20% hydrogen) or any other gas or mixture or means, such as a vacum, which can effect reduction.

The process limitations are quite flexible so long as firing to maturity, reduction and surface oxidation adjacent to the counterelectrode are obtained. Firing to maturity and reduction can be accomplished simultaneously or sequentially depending upon which conditions favor firm electrode adherence to the ceramic body for the particular electrode combination employed. Firing to maturity can be obtained at a temperature between about 1800" F. to about 2900 F. for from about 15 minutes to about 3 hours in either -an oxiding or a reducing atmosphere. If the body is fired to maturity in an oxidizing atmosphere first, then this must be followed by firing the unit in a reducing atmosphere until the ceramic body has the characteristics of a semiconductor throughout.

The final oxidation temperature depends upon the nature of the particular counterelectrode employed and the threshold temperature of both this electrode and that of the oxygen-impervious electrode. If silvery is employed, the final oxidation temperature must be below the melting point of this metal, i.e. about 1760 F. A practical limit for this oxidation step is from about 1000 F. to about 2900o F. for a period of about 1 minute to about 30 minutes. When a high melting point oxygen-pervasive electrode is employed it may be applied at the same time the oxygen-impervious electrode is applied.

The electrical circuit elements described may be in any convenient shape, e.g. disc shape, as shown, square, rectangular, tubular, etc. In addition to their utilitv as capacitors the subject elements may be employed as diodes or thermistors.

As is evident from the foregoing, the invention is not to be limited to formation of the rather specific illustrative devices. Modifications and variation as well as the swbstitution of equivalents may be made without departing from the spirit ofthe invention as defined in the appended What is claimed is:

1. An electrical circuit element comprising a ceramic body having an oxygen-impervious electrode and an oxy- .gen-pervasive counterelectrode, the surface of Said body underlying said counterelectrode being in an oxidized state and said body underlying said oxygen-impervious electrode being in a reduced state.

2. An electrical circuit element comprising a ceramic body having an electrode and a counterelectrode, the surface of said body underlying said counterelectrode being in an oxidized state and said body underlying said electrode being in a reduced state, said electrode being oxygen-impervious -at temperatures at which said counterelectrode is oxygen-pervasive.

3. An electrical circuit element comprising a ceramic titanate body having an electrode selected from the group consisting of the platinum group metals, gold and their oxygen-impervious alloys, and a counterelectrode of an oxygen-pervasive material, the surface of said body underlying the counterelectrode being in an oxidized state and said body underlying said electrode being in a reduced state.

4. A ceramic capacitor comprising a ceramic titanate body having a first electrode selected from the group consisting of the platinum group metals, gold and their oxygen-impervious alloys, and a counterelectrode of an oxygen-pervasive material, the surface of said body beneath said counterelectrode being in an oxidized state and having the characteristics of an insulator and said body underlying said first electrode being in a reduced state and having the properties of a semiconductor.

5. A ceramic capacitor comprising a ceramic titanate body having a first electrode of palladium alloyed with from 0 to 30% by weight of silver and a counterelectrode of an oxygen-pervious material, the surface of said body underlying the counterelectrode being in an oxidized state and said body underlying said first electrode being in a reduced state.

6. A ceramic capacitor comprising a barium titanate body having a first electrode of palladium and a counterelectrode of silver, the surface of said body underlying the counterelectrode being in an oxidized state and said body underlying the palladium electrode being in a reduced state.

7. A ceramic capacitor comprising a barium titanate body lhaving a first electrode of platinum and a counterelectrode of silver, the surface of said body underlying the counterelectrode being in an oxidized state and said 7 body underlying the platinum electrode being in a reduced state.

8. A ceramic capacitor comprising, a first oxygen-impervious electrode sandwiched between two ceramic titanate layers, two oxygen-pervasive counterelectrodes positioned on opposite surfaces of said titanate layers, said counterelectrodes being in electrical communication with one another, the titanate layers adjacent to said first electrode being in a reduced state and the surface of the titanate layers underlying said counterelectrodes being in an oxidized state.

9. A ceramic capacitor comprising a first electrode selected from the group consisting of the platinum group metals, gold and their oxygen-impervious alloys, sandwiched between two ceramic titanate layers, two oxygenpervasive counterelectrodes which are positioned on opposite surfaces of said titanate layers and which are in electrical communication with one another, the surface of the titanate layers underlying said counterelectrodes being in an oxidized state and the titanate layers adjacent to said first electrode being in a reduced state.

10. A ceramic capacitor comprising a first electrode of palladium alloyed with from to 30% silver, sandwiched between two ceramic titanate layers, two oxygen-pervasive counterelectrodes which are positioned on opposite surfaces of said titanate and which are in electrical communication with one another, the surface of the titanate layers underlying said counterelectrodes being in an oxidized state and the titanate layers adjacent to said rst electrode being in a reduced state.

11. A ceramic capacitor comprising a iirst electrode of palladium sandwiched between two ceramic titanate layers, two silver counterelectrodes which are positioned on opposite surfaces of said titanate and which are in electrical communication with one another, the surface of the titanate layers underlying said counterelectrodes being in an oxidized state and the titanate layers adjacent to said first electrode -being in a reduced state.

12. A ceramic capacitor comprising a rst electrode of platinum sandwiched between two ceramic titanate layers, two silver counterelectrodes which are positioned on opposite surfaces of said titanate and which are in electrical communication with one another, the surface of the titanate layers underlying said counterclectrodes being in an oxidized state and the titanate layers adjacent to said rst electrode being in a reduced state.

References Cited by the Examiner UNITED STATES PATENTS 2,119,115 5/1938 Rohnfeld 317-242 2,142,705 1/1939 Tarr 317-242 2,520,376 9/1950 Roup 317-258 X 2,627,645 2/ 1953 Harris 29-25.42 2,633,543 3/1953 Howatt 317-258 X 2,759,854 8/1956 Kilby 317-258 X y 2,899,345 8/1959 Oshby 317-258 X 2,915,808 10/1959 Clemons 29-25.42 3,042,846 7/1962 Lawson.

LEWIS H. MYERS, Primary Examiner.

R. K. SCI-IAEFER, E. GOLDBERG,

Assistant Examiners. 

1. AN ELECTRICAL CIRCUIT ELEMENT COMPRISING A CERAMIC BODY HAVING AN OXYGEN-IMPERVIOUS ELECTRODE AND AN OXYGEN-PERVASIVE COUNTERELECTRODE, THE SURFACE OF SAID BODY UNERLYING SAID COUNTERELECTRODE BEING IN AN OXIDIZED STATE AND SAID BODY UNDERLYING SAID OXYGEN-IMPERVIOUS ELECTRODE BEING IN A REDUCED STATE. 